1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a reflective liquid crystal display device and a fabricating method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a contrast ratio and an aperture ratio of the reflective liquid crystal display device.
2. Discussion of the Related Art
Flat panel display (FPD) devices processing and displaying a mass of information have been a subject of recent researches in the coming of the information age. Among the FPD devices, liquid crystal display (LCD) devices are developed as next generation display devices because they are portable and have a low power consumption. Presently, among the various types of LCD devices commonly used, active matrix LCD (AM-LCD) devices in which switching elements such as thin film transistors (TFTs) are disposed in a matrix for each pixel region have been developed because of their high resolution and superiority in displaying moving images.
In general, LCD devices are classified into transmissive LCD devices using a backlight unit and reflective LCD devices using an external light source. The transmissive LCD devices use a backlight unit, which consumes more than two thirds of the total power. On the other hand, since the reflective LCD devices use an external light source instead of a backlight unit, the power consumption is reduced. Accordingly, the reflective LCD devices are researched and developed due to their advantages over the transmissive LCD devices.
FIGS. 1A and 1B are schematic plane views showing an array substrate and a color filter substrate according to the related art, respectively.
In FIG. 1A, a gate line 14 and a data line 28 crossing each other are disposed on a first substrate 10. A thin film transistor (TFT) “T” is disposed at each intersection of the gate line 14 and the data line 28. A sub-pixel region “P” is defined by the gate line 14 and the data line 28 and a reflective electrode 38 connected to the TFT “T” is formed in the sub-pixel region “P”. The reflective electrode 38 overlaps the adjacent gate line 14 and the adjacent data line 28 to increase brightness. The reflective electrode 38 is formed of an opaque material having a high reflectance to reflect the incident light from the exterior. The reflective electrode 38 is a pixel electrode functioning as a reflecting plate. A region “L” of slant lines corresponds to a black matrix (not shown) of a second substrate (not shown).
In FIG. 1B, a second substrate 50 also has the sub-pixel region “P”. A black matrix 52 is formed at a border region between the adjacent sub-pixel regions “P”. Red, green, and blue color filters 54a, 54b, and 54c are sequentially disposed in each sub-pixel region “P”. The red, green, and blue color filters 54a, 54b, and 54c constitute a color filter layer 54 and a common electrode (not shown) is formed on the color filter layer 54. A region “M” surrounded by dotted lines corresponds to the reflective electrode 38 (shown in FIG. 1A).
FIG. 2A is a schematic cross-sectional view of a reflective liquid crystal display device having a mirror type reflective electrode and a front scattering film according to the related art, and FIG. 2B is a schematic cross-sectional view of a reflective liquid crystal display device having an uneven surface type reflective electrode according to the related art. FIGS. 2A and 2B are taken along line II—II of FIGS. 1A and 1B, respectively.
In FIG. 2A, first and second substrates 10 and 50 face into and are spaced apart from each other. A thin film transistor (TFT) “T” including a gate electrode 12, a semiconductor layer 22, and source and drain electrodes 24 and 26 is formed on the inner surface of the first substrate 10. A passivation layer 36 including a drain contact hole 30 is formed on the TFT “T” and the drain contact hole exposes the drain electrode 26. A reflective electrode 38 connected to the drain electrode 26 through the drain contact hole 30 is formed on the passivation layer 36 in a sub-pixel region “P”. A data line 28 is connected to the source electrode 24. To show an overlapping structure of the reflective electrode 38 and the data line 28, a data line of the adjacent sub-pixel region is also illustrated. The passivation layer 36 is formed to be thick using an insulating material having a low dielectric constant to reduce an electrical interference between the data line 28 and the reflective electrode 38. The reflective electrode 38 is formed of an opaque metallic material having a high reflectance. Since the reflective electrode 38 has a flat surface, the light is reflected as if the reflective electrode 38 is a mirror. This phenomenon is referred to as a mirror reflection.
A black matrix 52 is formed on the inner surface of the second substrate 50 in a region corresponding to the TFT “T” and the data line 28. A color filter layer 54 is formed on the black matrix 52, and a common electrode 56 is formed on the color filter layer 54. A liquid crystal layer 70 is interposed between the reflective electrode 38 and the common electrode 56. A front scattering film 58 including a plurality of scattering particles 59 is formed on the outer surface of the second substrate 50. The front scattering film 58 scatters the light reflected from the reflective electrode 38 to increase a reflection efficiency. An LCD device having a front scattering film provides a reflection efficiency higher than an LCD device having a mirror type reflective electrode. However, the LCD device having a front scattering film has some disadvantages in that a material for the front scattering film is expensive and it is difficult to adjust reflectance. To resolve these problems, an LCD device including a reflective electrode of an uneven shape without a front scattering film has been suggested.
In FIG. 2B, descriptions for portions having the same structures as the LCD device of FIG. 2A will be omitted for simplicity. A passivation layer 96 having a drain contact hole 90 and a first uneven pattern in a sub-pixel region “P” is formed on a TFT “T”. The drain contact hole exposes a drain electrode 86. A reflective electrode 98 connected to the drain electrode 86 through the drain contact hole 90 is formed on the passivation layer 96. The reflective electrode 98 includes a second uneven pattern corresponding to the first uneven pattern. Since the reflective electrode 98 is formed by depositing a metallic material on the passivation layer 96, the second uneven pattern is formed on the first uneven pattern. A region “N” indicates the second uneven pattern formed by using the first uneven pattern as a seed. As compared to an LCD device including a front scattering film, a material cost is reduced due to the omission of a front scattering film, and a process efficiency is improved due to an adjustment of reflectance by controlling the structure of an uneven pattern in an LCD device having an uneven surface type reflective electrode.
FIG. 3 is a schematic cross-sectional view showing a path of an incident light in a liquid crystal display device having an uneven surface type reflective electrode according to the related art.
In FIG. 3, first and second substrates 110 and 130 face into and are spaced apart from each other. A liquid crystal layer 120 is interposed between the first and second substrates 110 and 130. An incident light “L1” from the air passes through the second substrate 130 and the liquid crystal layer 120, and is reflected at a reflective electrode 112 having an uneven pattern. Then, the reflected light is emitted through the second substrate 130 and is perceived by users. For example, when the incident light “L1” has an incident angle α (alpha) of about 30° with respect to the normal direction of the second substrate 130, a refracted light “L2” has a refraction angle β (beta) of about 20° with respect to the normal line according to Snell's Law. The refracted light “L2” passes through a liquid crystal layer 120 and is reflected by the reflective electrode 112. A slanting angle θ (theta) due to the uneven pattern of the reflective electrode 112 may be within the range of about 6° to 10° so that reflected light “L3” can be transmitted within a main viewing angle γ (gamma). The slanting angle θ (theta) due to the uneven pattern can be adjusted during a fabricating process of the reflective electrode 112 having the uneven pattern.
FIGS. 4A and 4B are schematic cross-sectional views showing the fabricating process of an uneven pattern having a single layer structure according to the related art.
In FIG. 4A, organic material patterns 152 are formed on a substrate 150 by depositing and patterning an organic material. The organic material patterns are overlapped or spaced apart from each other. The organic material is formed of a positive type photosensitive material and its irradiated portion is eliminated by development. The organic material patterns 152 have a rectangular shape due to characteristics of the photolithographic process.
In FIG. 4B, the organic material patterns 152 (shown in FIG. 4A) are melted and cured by a heat-treatment to become uneven patterns 153 having an effective slanting angle θ (theta). A straight sidewall of each organic material pattern 152 (shown in FIG. 4A) becomes round by using the spread of materials during the melting process and the melted uneven patterns 154 become hard during the curing process. The effective slanting angle θ (theta) can be controlled through the adjustment of a distance between the organic material patterns 152 (shown in FIG. 4A) and the area of an overlapped portion. However, the fabricating process of the uneven patterns 154 becomes complicated and provides a low reproducibility.
FIGS. 5A to 5C are schematic cross-sectional views showing the fabricating process of an uneven pattern having a double layer structure according to the related art.
In FIG. 5A, organic material patterns 162 spaced apart from each other are formed on a substrate 160. The organic material patterns 162 have a rectangular shape through the same photolithographic process as that of the organic material patterns 152 of FIG. 4A.
In FIG. 5B, the organic material patterns 162 (shown in FIG. 5A) are melted and cured by a heat-treatment to become uneven patterns 164. The uneven patterns 164 are used as seeds for the subsequent uneven shape.
In FIG. 5C, an organic material layer 166 having an uneven shape is formed on the uneven patterns 164 by coating an organic material. An effective slanting angle θ (theta) can be adjusted by a space between the adjacent uneven patterns 164. Since each uneven pattern 164 has an equal shape, a density of the uneven patterns 164 can be controlled by the adjustment of the space along the main viewing angle. Thus, the uneven surface of the organic material layer 164 can be obtained by using an acrylic organic material through two step processes to increase the reflectance along the main viewing angle. In the first step process, seeds of an uneven shape are randomly formed. In the second step process, a resulting uneven pattern is obtained by forming an organic material layer on the seeds with various thicknesses.
Generally, the organic material layer 166 on the uneven patterns 164 is formed of a transparent organic material. In the reflective LCD device according to the related art, metal lines such as a gate line and a data line are exposed between the reflective electrodes. Since a voltage (i.e., data signal) is not applied to a liquid crystal layer over the metal lines, the reflected light from the metal lines degrades a contrast ratio of the reflective LCD device. To solve this problem, a black matrix is formed on the facing substrate. However, an aperture ratio is reduced due to the increase of a black matrix area. Moreover, since the black matrix is formed on the facing substrate, a contrast ratio is reduced when a misalignment occurs during an attaching process.